Definition of Pelizaeus-Merzbacher disease

Pelizaeus-Merzbacher disease: A disorder of the central nervous system (CNS) in which there is loss of myelin, the sheath around the nerves. The disease is clinically characterized by nystagmus (rhythmical oscillation of the eyes), impaired motor development, tremor, progressive spasticity (increased muscle tone), ataxia (wobbliness), choreoathetotic movements, and dysartria (difficulty speaking). Pelizaeus-Merzbacher disease (PMD) in its classical form manifests in infancy or early childhood and progresses to severe spasticity and ataxia. The lifespan may be shortened.

PMD is due to mutation in the gene PLP1. This gene is located on the X chromosome in band Xq22. The disease describes an X-linked pattern of inheritance with boys who have the mutation affected with the disease while females with the mutation are carriers.

The PLP1 gene encodes proteolipid protein (PLP), the most abundant protein of the myelin sheath in the CNS. The mutation in PLP1 in PMD results in loss of myelin and that, in turn, causes the neurological abnormalities.

The severity of myelin loss is dependent on the particular PLP1 mutation and can range from early lethal forms of PMD to a mild disorder known as spastic paraplegia type 2 (SPG2).

Among the mutations in the PLP1 gene locus that can cause PMD is a duplication of PLP1 in which the duplicated region may be far away from the original PLP locus in chromosome region Xq22. The PLP1 duplication is almost always present in the mothers of affected boys and usually can be traced to the maternal grandfather.

The disease is named for the German neurologist Friedrich Pelizaeus (1850-1917) and psychiatrist Ludwig Merzbacher (1875-1942) who independently described the disorder in 1885 and 1909, respectively. The disease is a hypomyelinating X-linked leukodystrophy.